1. Field of the Invention
The present invention relates to the field of fabricating mesoscopic structures.
2. Prior Art
Considerable effort over the past two decades has been expended to develop techniques to fabricate mesoscopic materials (materials that exhibit unique quantum mechanical effects due to their ultrasmall dimensions but are still considered large on an atomic scale), usually embedded in a protective or interactive matrix. The constraints on size are highly dependent upon the specific application. However, the ultimate goal is to produce materials that can be engineered from macroscopic dimensions to atomic proportions in size. The quantum mechanical properties of mesoscopic materials are dominated by the effects of size confinement and, if the particles are close enough to each other, cooperative effects.
Quantum confinement refers to materials that have spatial dimensions small enough to exhibit properties which differ from those of the bulk material according to the laws of quantum mechanics. Examples of quantum confinement in semiconductor materials are quantum wells (confinement in one dimension), quantum wires (confinement in two dimensions) and quantum dots (confinement in three dimensions). Quantum confined systems have generated considerable interest in both basic research and commercial applications because of their unique electronic and optical properties and their potential for ultra small devices.
Cooperative effects between quantum confined systems refers to additional effects exhibited when quantum confined systems interact. A variety of methods are currently employed to fabricate such systems. However, only quantum well devices have met with any great success. Previous work on ordered arrays of quantum dots and wires that exhibit cooperative effects have suffered from a lack of uniformity in the array over length scales considered large compared to the size of an element in the array. Since the electronic properties, and hence the optical properties, of quantum confined systems generally change rapidly as a function of size and separation, this lack of uniformity tends to wash out any features due to cooperative effects. In addition, any enhancement gained from large numbers of identical particles will also be washed out. The variance in size and geometry also tends to make electronic device performance unpredictable and greatly reduces optical device efficiencies.
Devices that utilize tunneling of electrons or elementary excitations such as excitons, are very sensitive to barrier heights and widths. Arrays of tunneling devices that require identical barriers (i.e. identical device characteristics) then become extremely sensitive to the overall structure of the array in terms of the element dimensions, barrier widths, and positioning of the elements within the array.
Recent attempts at defining quantum confined arrays of wires or dots have relied on a variety of masking, etching and growth techniques. Due to the small size requirements for quantum confinement (often&lt;50 nm), these processes usually contaminate the quantum confined material by damage or extraneous material deposition, which is generally detrimental to device performance. Where material damage is intentionally invoked to define the structure of the device, component boundaries tend to be vague, compromising large scale geometries. Arrays fabricated by controlled growth techniques over substrates, tend to be extremely labor and time intensive, and exhibit good uniformity over a relatively small region of the array. Chemical etching of free standing arrays utilizing masking techniques tend to be fragile and curve definition is extremely process and material dependent.
Supersaturation growth techniques are capable of producing very small microcrystalites (usually not greater than 10 nm) of II-VI and I-VII semiconductor material in glass hosts. Although these quantum dot systems are usually of excellent optical quality, the dots exhibit a random distribution of particle sizes suspended within the medium. In addition, there is no control over the position of individual particles. To date, they have found extensive use as optical color filters and have demonstrated an optical nonlinearity. However, they are of no immediate utility to the electronics industry.